This project is focused on the development of gene therapy for Wiskott-Aldrich syndrome (WAS), a severe immunodeficiency disorder also characterized by a low platelet count and chronic eczema. Affected boys may also suffer from autoimmunity and/or develop a neoplasm. We have made the following advances which support the application of gene transfer into blood stem cells for treatment of WAS: 1) developed a novel, HIV1 based lentiviral vector system to facilitate gene transfer into stem cells; 2) identified the envelope protein from Vesicular Stomatitis Virus (VSV-G) as providing the highest efficiency of gene transfer into repopulating cells; and 3) developed a methodology for deriving stable producer clones that will facilitate vector preparation for our planned clinical trial. In Specific Aim 1, experiments are proposed to identify a lentiviral vector design that achieves normal expression of Wiskott-Aldrich syndrome protein (WASp) in hematopoietic cells and demonstrates therapeutic potential, in Sub-Aim 1.1, we will compare the levels of WASp expression achieved with various promoters to select one for use in our clinical trial. In a second exploratory sub-aim of Specific Aim 1, we will map and functionally characterize distant tissue specific regulatory elements that may influence WASp gene expression. In Specific Aim 2, we will evaluate the safety of WASp clinical vector using 2 cellular assays that detect proto-oncogene activation. Vectors will be assayed for their potential to activate the LM02 proto-oncogene in Jurkat T-celis and also for their ability to induce myeloid immortalization of primary lineage depleted bone marrow cells. In Specific Aim 3, we propose to evaluate lentiviral vector mediated WASp gene transfer in WAS patients. Clinical vector design will be determined by the functional studies proposed in Sub-Aim 1.1 as well as the cellular assays for protooncogene activation in Specific Aim 2. Eligible participants are those whose platelet count is < 50,000/mm3 who have other significant clinical manifestations of WAS but lack a matched related or unrelated allogeneic stem cell donor. G-CSF mobilized peripheral blood stem cells will be transduced and returned to participants following myelosuppressive therapy with Busulfan. Safety and feasibility will be assessed within 2 months of infusion of transduced cells and the protocol amended if one or more stopping rules are met. Objective measures of efficacy include a progressive increase in the number of genetically modified cells, particulariy in the lymphoid lineages, an increase in platelet count to &50,000/mm3 and a return of IgM levels to normal by 1 year. Patients will be observed long-term for restoration of immune function and for any evidence of vector induced clonal dominance or neoplasia.